The present invention relates generally to semiconductor device manufacturing and, more particularly, to a niobium thin-film stress relieving layer for thin-film solar cells.
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. Generally, p-n junction based photovoltaic cells include a layer of an n-type semiconductor in direct contact with a layer of a p-type semiconductor. When a p-type semiconductor is positioned in intimate contact with an n-type semiconductor, a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (the p-type side of the junction). However, the diffusion of charge carriers (electrons) does not happen indefinitely, as an opposing electric field is created by this charge imbalance. The electric field established across the p-n junction induces a separation of charge carriers that are created as result of photon absorption.
The most common type of solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is still higher than the cost of electricity generated by the more traditional methods. Since the early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost, thin-film growth techniques that can deposit solar cell quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
The increased interest in thin-film photovoltaics has been due primarily to improvements in conversion efficiency of cells made at the laboratory scale, with the anticipation that manufacturing costs can be significantly reduced compared to the older and more expensive crystalline and polycrystalline silicon technology. The term “thin-film” is thus used to distinguish this type of solar cell from the more common silicon based cell, which uses a relatively thick silicon wafer. While single crystal silicon cells still demonstrate the best conversion efficiency to date at over 20%, thin-film cells have been produced which can perform close to this level. As such, performance of the thin-film cells is no longer the major issue that limits their commercial use. Instead, primary factors now driving the commercialization of thin-film solar cells include cost, manufacturability, reliability and throughput, for example.